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bridgeport's  improved 
Sewerage  System 


BY 

•o 


JAMES  A.  MCELROY 

CITY  ENGINEER.  BRIDGEPORT.  CONN. 


Reprint  From  The 

Proceedings  of  the  Connecticut  Society  of  Civil  Engineers 

Nineteen  Hundred  and  Twenty 


BRIDGEPORT    SEWERAGE    SYSTEM.  53 


BRIDGEPORT'S  IMPROVED  SEWERAGE 
SYSTEM. 

By  James  A.  McElroy,  City  Engineer,  Bridgeport,  Conn. 

In  1906  the  Common  Council  of  the  City  of  Bridgeport  author- 
ized the  mayor  to  appoint  a  special  commission  to  investigate  the 
existing  sewer  conditions  and  formulate,  if  possible,  a  new  sewer 
system  to  meet  the  present  and  future  needs  of  the  City.  The 
well  known  firm  of  Hering  &  Fuller  was  secured  by  the  com- 
mission, and  their  report,  submitted  in  the  spring  of  1908,  has 
served  as  a  guide  for  all  sewers  built  since  that  time. 

In  the  past  all  sewers  in  Bridgeport  had  been  built  on  the  com- 
bined system,  that  is,  the  same  sewers  carried  both  storm  water 
and  house  sewage.  These  sewers,  many  of  which  were  too 
small,  emptied  into  the  nearest  tide  water.  The  Pequonnock 
River  which  runs  through  the  center  of  the  city  and  naturally 
divides  the  city  into  two  districts,  which  are  now  known  as  the 
Eastern  and  Western  Sewage  Districts,  had  about  26  sewers 
emptying  into  it,  while  Ash  Creek  which  runs  along  the  western 
edge  of  the  city  had  4,  and  Johnsons  Creek,  the  eastern  boundary, 
had  12.  There  were  8  sewers  flowing  into  Yellow  Mill  Pond,  in 
the  center  of  the  eastern  district  and  9  into  Cedar  and  Bun- 
Creek  in  the  western  district. 

It  was  realized  that  the  city  could  not  continue  indefinitely 
emptying  sewage  into  these  small  streams  some  of  which  were 
hardly  more  than  mud  flats  at  low  tide,  and  that  at  some  time  in 
the  future  it  would  all  have  to  be  collected  and  discharged  into 
the  sound  through  long  outfalls,  either  with  or  without  treatment. 

The  topography  of  Bridgeport  is  such  that  large  areas  near  the 
sound  are  very  low,  and  the  Pequonnock  River  and  other  streams 
extend  so  far  into  the  land  at  tidewater  level,  that  it  would  be 
impossible  to  collect  the  sewage  and  deliver  it  either  to  outfalls  or 
disposal  work  without  pumping.  To  collect  large  quantities  of 
storm  water  in  combined  sewers  and  then  pump  it,  would  add 
greatly  to  the  cost  of  construction  of  the  trunk  sewers  and  to  the 
pumping  cost,  so  Hering  &  Fuller  recommended  separate  sewers 
for  rainwater  wherever  practicable,  with  storm  water  overflows 


54 


THIRTY-SIXTH    ANNUAL    MEETING. 


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STARS  SHOW  OLD  SEWER  OUTLETS. 


BRIDGEPORT    SEWERAGE    SYSTEM. 


55 


INTERCEPTORS  AND  PUMPING  STATION. 


56  THIRTY-SIXTH    ANNUAL    MEETING. 

in  some  of  the  older  sections  of 'the  city  where  the  combined  sys- 
tem would  have  to  be  retained. 

They  considered  the  city  naturally  divided  into  two  districts 
by  the  Pequonnock  River,  and  planned  a  pumping  station  and 
system  of  intercepting  sewers  for  each  district.  It  was  their 
opinion  that  by  carrying  the  outfall  to  deep  water,  i.  e.,  20  feet 
to  25  feet,  no  further  treatment  would  be  required;  but  sug- 
gested that  when  sites  were  acquired  for  the  pumping  stations, 
sufficient  land  be  secured  to  enable  the  city  to  build  treatment 
plants,  if  desired  at  some  future  time. 

By  1916  practically  all  the  interceptors  had  been  completed 
down  as  far  as  the  sewage  would  discharge  by  gravity.  No  fur- 
ther extensions  of  the  interceptors  could  be  made  until  the  pump- 
ing stations  were  built.  The  firm  of  Hering  &  Gregory, 
successors  to  Hering  &  Fuller,  were  asked  to  review  the  work 
done  to  date  and  recommend  a  program  for  the  future.  They 
considered  the  advisability  of  bringing  the  sewage  of  the  entire 
city  to  one  treatment  plant  located  on  Steeplechase  Island,  but 
finally  decided  in  favor  of  the  original  plan  of  Hering  &  Fuller 
which  divided  the  city  into  two  districts  with  a  complete  plant  for 
each  district. 

As  the  Western  District  was  the  more  important  it  wTas  decided 
to  finish  that  first;  so  contracts  were  let  for  the  construction  of 
the  Railroad  Avenue  and  Bostwick  Avenue  interceptor,  and  plans 
were  prepared  for  the  Western  District  Pumping  Station  at  the 
foot  of  Bostwick  Avenue. 

The  consulting  engineers  recommended  "an  Imhoff  tank  sewage 
treatment  plant  and  designed  the  pumping  station  accordingly. 
But,  although  the  city  did  not  intend  to  build  the  treatment  works 
at  this  time,  before  letting  the  contract  for  the  pumping  station 
it  was  decided  to  investigate  sewage  treatment  in  other  cities. 
So  a  committee  consisting  of  the  City  Engineer,  the  Director  of 
Public  Works,  the  President  of  the  Paving  and  Sewer  Commis- 
sion and  .the  chairman  of  the  committee  on  sewers  of  the  common 
council,  visited  a  number  of  plants  including  Fitchburg,  Wor- 
cester, Rochester,  Brooklyn,  Baltimore,  Plainfield  and  Wildwood, 
N.  J.  It  was  the  opinion  of  a  majority  of  this  committee  that 
the  Riensch-Wurl  fine  screen  method  of  sewage  treatment  was 
the  most  appropriate  system  for  Bridgeport. 


R  k  I DG  E  P( )  R  T    S  E  W  E  RAGE    SYSTE  M . 


57 


58 


THIRTY-SIXTH    ANNUAL    MEETING. 


BRIDGEPORT    SEWERAGE    SYSTEM.  59 

The  report  of  this  committee  was  submitted  to  two  prominent 
sanitary  engineers,  George  W.  Fuller  and  George  A.  Johnson, 
for  their  advice.  They  approved  of  the  committee's  report ;  so 
new  plans  were  prepared  for  the  Western  District  pumping 
station,  including  screening  and  pumping  equipment  to  take  care 
of  the  present  sewage  of  the  district,  with  provisions  for  the 
installation  of  additional  equipment  when  required  in  the  future. 

The  principal  reasons  which  led  the  committee  to  decide  on 
the  fine  screen  rather  than  the  Imhoff  tank  method  of  treatment 
was  the  uncertainty  of  the  action  of  Imhoff  tanks.  It  was  noted 
that  several  Imhoff  plants,  designed  by  competent  engineers,  were 
unsatisfactory,  while  few  of  the  plants  which  were  giving  satis- 
factory service  were  handling  the  amount  of  sewage  for  which 
they  were  designed.  As  Bridgeport's  Western  Treatment  Plant 
was  to  be  located  on  the  southwest  edge  of  a  well  populated  dis- 
trict, with  the  prevailing  winds  coming  from  the  southwest,  it 
was  thought  best  not  to  run  the  risk  of  having  another  such 
experience  as  they  had  in  Plainfield,  where  they  have  finally  had 
to  install  a  Riensch-Wurl  fine  screen  ahead  of  their  Imhoff  tanks. 

The  original  Hering  &  Fuller  report  recommended  disposal  of 
Bridgeport  sewage  by  dilution.  The  process  finally  adopted  was 
that  of  clarification  by  fine  screens,  followed  by  dilution.  If  fur- 
ther treatment  is  desired  at  some  future  time,  the  outfall  can  be 
extended,  or  if  necessary  additional  treatment  works  can  be  built 
on  land. 

The  flow  line  of  the  interceptor  at  the  plant  is  at  elevation  ioo, 
22  feet  below  the  street  level  and  10  feet  below  mean  low  water. 
Several  combinations  of  screens  and  pump  were  studied  includ- 
ing the  plan  of  having  high  level  screens  following  the  pumps,  so 
that  the  screened  effluent  would  flow  by  gravity  to  the  outfall. 
This  scheme  would  have  saved  something  on  the  cost  of  construc- 
tion of  the  station,  but  it  was  discarded  mainly  because  the 
screens  would  be  more  effective  if  installed  ahead  of  the  pumps. 

In  the  plant  finally  decided  upon,  the  sewage  from  the  71 -inch 
interceptor  flows  through  the  sluice  gate  to  a  bar  screen  at  eleva- 
tion 100,  thence  through  a  concrete  channel  to  the  R-W  Screens. 
The  screened  effluent  flows  over  an  adjustable  weir  to  the  sump 
at  elevation  80  and  is  then  lifted  by  the  pumps  to  the  force  main 
at  elevation  105. 


6o 


THIRTY-SIXTH    ANNUAL    MEETING. 


BRIDGEPORT    SEWERAGE    SYSTEM. 


6  i 


62  THIRTY-SIXTH    ANNUAL    MEETING. 

The  plant  is  designed  for  three  Riensch-Wurl  screens,  to  han- 
dle a  total  dry-weather  flow  of  35  million  gallons  per  day.  The 
estimated  population  of  the  district  when  fully  developed  is 
180,000.  As  the  present  population  of  the  district  is  in  the 
neighborhood  of  100,000,  two  screens  will  be  installed  now  and 
the  third  one  when  needed.  The  R-W  Screen  consists  of  a 
circular  disc  22  feet  in  diameter  inclined  at  an  angle  of  15  degrees 
to  the  horizontal.  A  truncated  cone  12  feet  in  diameter,  at  the 
base,  is  centrally  mounted  on  the  disc.  The  surface  of  both  disc 
and  cone  consists  of  a  number  of  removable  bronze  plates,  per- 
forated with  slots  two  inches  long  and  3/64  of  an  inch  wide. 
The  lowest  point  of  the  disc  is  set  at  the  elevation  of  the  influent, 
and  the  slope  is  such  that  about  one-third  of  the  disc  will  be  above 
the  surface  of  the  water. 

The  sewage  flows  on  to  the  screen  which  is  revolving  at  a  rate  of 
about  one  revolution  in  two  minutes.  The  liquid  passes  through 
the  perforations  in  the  disc  and  flows  over  the  weir  into  the  pump 
sump.  The  suspended  solids,  which  are  retained  on  the  screen 
are  lifted  out  of  the  water  as  the  screen"  revolves,  and  are  brushed 
off  the  screen  into  the  hopper  of  the  screening  conveyor  system. 
The  brushes  are  cylindrical  and  are  carried  on  a  revolving  spider 
which  is  mounted  on  a  shaft  parallel  with  the  main  disc  shaft. 
These  brushes  work  on  the  same  principle  as  a  street  sweeper, 
which  is  carried  along  the  street  mounted  on  a  truck  and  at  the 
same  time  is  revolving  and  brushing  the  refuse  ahead  of  it. 

The  screenings  are  conveyed  from  the  hopper  to  the  storage 
tanks  by  a  pneumatic  system  which  does  away  with  the  necessity 
of  handling  it  in  open  cans  or  on  an  open  conveyor-belt.  From 
the  storage  tank  which  is  located  10  feet  above  the  street  level, 
the  screenings  will  be  dropped  into  tank  wagons,  carted  to  the 
town  farm  and  buried. 

The  pumping  equipment  consists  of  two  20-inch  horizontal  cen- 
trifugal pumps  each  having  a  capacity  of  15  million  gallons  per 
day  and  two  15-inch  horizontal  centrifugal  pumps  with  a  capacity 
of  eight  million  gallons  per  day  each.  These  pumps  will  be 
direct  connected  to  electric  motors,  controlled  by  floats  in  the 
pump  sump.  The  floats  are  adjustable  and  electrically  connected 
to  switches  and  automatic  motor  starters,  mounted  on  panels  of 
the  main  switch  board. 


BRIDGEPORT    SEWERAGE    SYSTEM. 


63 


64 


THIRTY-SIXTH     ANNUAL    MEETING. 


BRIDGEPORT    SEWERAGE    SYSTEM.  65 

A  Venturi  tube  is  to  be  installed  on  the  outfall,  with  the  record- 
chart  and  register  located  on  one  of  the  main  switch  board 
panels.  The  60-inch  gate  valve,  electrically  operated,  will  be 
installed  in  a  gate  house  at  the  discharge  end  of  the  venturi  tube. 

The  building  will  contain  a  complete  system  of  heating,  ven- 
tilating and  plumbing,  including  toilets  and  shower  baths. 

The  State  Board  of  Health  approved  of  the  plans,  for  the  plant 
itself,  leaving  the  question  of  the  location  of  the  outfall  to  he 
decided  later,  so  construction  was  started  about  the  first  of  last 
year,  1919.  The  substructure  is  a  reinforced  concrete  caisson 
80  feet  inside  diameter  and  45  feet  deep.  The  cylindrical  shell 
forming  the  outside  wall  is  constructed  of  a  series  of  rings  ten 
feet  high.  The  rings  were  cast  above  ground  and  after  the  con- 
crete in  each  ring  had  properly  set,  the  forms  were  removed  and 
the  ring  sunk  until  the  top  was  approximately  a  foot  above 
ground.  The  forms  were  then  re-assembled  and  the  next  sec- 
tion cast.  When  the  caisson  had  reached  the  predetermined 
depth  the  bottom  was  placed  under  water.  After  the  concrete 
bottom  had  set,  the  caisson  was  unwatered,  the  bottom  made 
smooth  and  finished  to  a  surface.  This  part  of  the  work  is  fin- 
ished and  the  inside  walls,  partitions,  beams  and  floor  slabs  are 
now  being  built.  The  rectangular  superstructure  will  be  con- 
structed of  tapestry  brick  with  stone  trimmings  and  a  red  tile 
roof. 

The  lowest  ring,  which  contains  475  cu.  yds.  of  1-2-4  con_ 
crete  and  thirty  tons  of  steel,  was  completed  in  March  of  last 
year.  After  a  few  days  the  forms  were  removed  and  set  for  the 
second  ring.  The  same  forms  were  used  for  all  five  rings.  The 
second,  third  and  fourth  rings,  each  containing  500  cu.  yds.  of 
concrete  and  16  tons  of  steel,  were  finished  by  June  ist,  the  con- 
crete being  poured  in  continuous  operations  of  about  eight  hours 
for  each  ring. 

After  the  first  ring  had  been  poured  and  the  forms  removed 
the  ring  was  allowed  to  sink  by  excavating  the  material  from  the 
interior.  Two  three-quarter  yard  clam-shell  buckets  on  two  65- 
foot  boom  stiff-leg  derricks,  set  up  on  opposite  sides  of  the  cais- 
son, averaged  about  200  cu.  yds.  of  excavation  a  day.  One 
derrick  was  placed  on  the  north  side  of  the  caisson,  the  other  on 


66 


THIRTY-SIXTH    ANNUAL    MEETING. 


BRIDGEPORT    SEWERAGE    SYSTEM. 


67 


FORMS  IN  PLACE  FOR  THIRD  RING.      SHOWING  PUMPS. 


68  THIRTY-SIXTH    ANNUAL    MEETING. 

the  south.  On  the  east  side,  towards  Bostwick  Avenue  were  two 
one-yard  mixers  and  an  8o-foot  double  hoisting  tower  from 
which  the  concrete  could  be  delivered  through  chutes  to  any  part 
of  the  ring.  Just  north  of  the  tower  was  another  derrick  which 
supplied  gravel  and  sand  to  hoppers  located  above  the  mixers. 
As  the  excavation  proceeded  levels  were  taken  at  frequent  inter- 
vals each  day  on  the  top  of  the  shell  and  if  one  side  was  found  to 
be  sinking  too  fast,  excavation  was  stopped  on  that  side  and  con- 
tinued near  the  other  side  until  the  ring  was  level  again.  At  no 
time  was  the  caisson  more  than  nine  inches  out  of  level.  At  first 
it  was  not  intended  to  keep  the  caisson  dry  during  excavation,  but 
as  the  work  proceeded  and  it  was  seen  that  there  was  not  much 
water  coming  in,  pumps  were  installed  and  the  interior  was  kept 
dry  during  this  part  of  the  work. 

Several  of  the  engineers  who  looked  over  the  job  before  the 
contract  was  let  said  it  would  be  almost  impossible  to  sink- this 
caisson,  as  the  skin  friction  on  the  outside  would  hold  it  up.  As 
a  matter  of  fact  it  went  down  so  easily  that  the  top  ring  was  left 
off  until  the  caisson  was  within  three  feet  of  the  bottom. 
Progress  had  been  so  satisfactory  up  to  this  point  that  an  attempt 
was  made  to  excavate  still  further  and  try  to  lay  the  bottom  in 
the  dry.  It  was  found,  however,  that  the  pumps  were  drawing  too 
much  sand,  which,  flowing  in  from  the  outside  under  the  bottom 
edge  of  the  caisson  was  undermining  the  derricks  and  concrete 
tower.  So  the  top  ring  was  put  on,  the  caisson  was  allowed  to 
fill  with  water,  and  the  excavation  was  continued  until  the  ring- 
was  down  to  the  predetermined  depth  and  soundings  showed  that 
the  excavation  for  the  bottom  was  completed. 

The  bottom  was  then  put  in  under  water  in  one  continuous 
operation.  The  same  gang-  worked  right  through  and  put  in  1800 
cu.  yds.  of  concrete  in  thirty-nine  hours.  The  men  were  allowed 
to  knock  off  occasionally  for  rest  and  refreshment,  but  the  mixers 
were  kept  going  continuously  except  when  shut  down  for  minor 
repairs.-  There  was  a  two  hour  delay  about  noon  of  the  first 
day.  and  the  plant  was  shut  down  for  about  five  hours  at  different 
times  during  the  second  day. 

The  bottom  was  allowed  to  stand  three  weeks,  then  the  water 
was  pumped  out.     No  leaks  were  found  so  the  floor  was  finished 


BRIDGEPORT    SEWERAGE    SYSTEM. 


69 


7° 


THIRTY-SIXTH    ANNUAL    MEETING. 


BRIDGEPORT    SKWKK'.V.K    SYSTF.M.  7  I 

and  work  was  commenced  on  the  forms  for  the  inside  walls  and 
partitions. 


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